Reporter: It's the latest hollywood weight loss craze causing a hoopla. It's called hoopnotica. A favorite of kelly osbourne. You want to look good, you want to feel good. Reporter: She did to it get ready for oscar night. It's a secret to keeping 70 pounds off, strengthening her back and arms and trimming two frinchs her weight. Of about 50 pounds. Reporter: Jackie is a certified instructor and gave me a quick lesson in venice, california. Elbows in, hands supporting. We're going to wac to the side and spin it. Fwa forward and back. Get your arms out of the way. Reporter: It's transformed into a cardio workout. It is cardio. And strength you're using the core. I feel it from my heels up to my armpits. This is awesome. Reporter: Gabriella started it. She dropped 50 pounds in a year after weighing 182. Burn up to 60 calories an hour. Moving your arms in and out and coming up. The. Reporter: What do you feel is working out? My legs. Abs. You to this and learn this and all of a sudden, your husband is like, what is she doing? Reporter: It all depends on how she spins it. For "good morning america," rachel smith, venice, california. And here to join us and show us hoopnotica is jackie berk. Welcome to "good morning america." It's a little rainy out. I'll use this. Kelly lost two inches from waist in one week? In one week leading up to the golden globes. They had to take in her dress in an emergency operation. That sounds serious. It's a good problem to have. Why does it work so well? It burns on average 420 calories an hour. It's the core but a full-body workout. I'll show you calm of things you can do to up the ante on it. Perfect. Sam, come on down here. The biggest key is to start -- come on, sam. We got this. Go, jackie. Start with a hoopnotica hoop. Get it on the website. You want a nice big hoop. It's inches a well as pounds. Right. Exaggerate your pushes. Sam -- do you think -- spencer. Just exaggerate your pushes and everything will be fine. That's fantastic. Look at you. And from here, we can add in some cardio exercise. Come on. Can't do it. What piece offed a vaseld you give girls at home? Grab a hope, we're availab to answer any questions in terms of sizing. Get a dvd. Do it at home. Or find a class. Maybe yours is not the right size. Maybe that's it. We want to thank jackie. Sam, give it one more try. No, I'm not doing it.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Amy Adams Juggles Two Men and Two Accents in 'American Hustle'

Now Playing: {{itm.title}}

{"id":19065111,"title":"Kelly Osbourne's Hula-Hoop-Related Weight-Loss Secret","duration":"3:41","description":"Star reveals a new slimmed down look on the latest cover of Self magazine.","url":"/GMA/video/kelly-osbourne-weight-loss-hulu-hoop-related-weight-19065111","section":"GMA","mediaType":"default"}